The invention relates to a continuous insulation monitor comprising an input voltage generator, either DC or of a preset frequency different from the frequency of the mains system to be monitored, a processing circuit comprising a measuring circuit of the DC or preset frequency component of the voltage applied to its terminals, a microprocessor comprising a first input connected to the output of the measuring circuit and comprising means for converting the output signal of the measuring circuit into a numerical value, for computing from the numerical value a value representative of the insulation resistance of the mains system to be monitored, for comparing the value representative of the insulation resistance with at least one preset threshold and for supplying an alarm signal on an output when this value is lower than the threshold, and an alarm and signalling circuit connected to the microprocessor output.
In an insulation monitor of this kind, to obtain a good precision for all insulation resistance values, an analog-to-digital converter has to be used providing a greater precision than the required precision, for example a 12-bit converter.